1. Field of the Invention
The present invention relates to a constant voltage regulator that applies a constant voltage to a load, a method of controlling the regulator, and an electric device provided with the regulator.
2. Description of the Prior Art
Each of the conventional constant voltage regulators is provided with a driver transistor for applying a constant voltage to a load. And, an amount of a control current for turning on this driver transistor is limited when the constant voltage regulator is overloaded, for example, when an output terminal of the regulator is short-circuited, thereby the constant voltage can be applied to the load even after the load increases excessively.
FIG. 7 shows an explanatory circuit diagram of a conventional constant voltage regulator. In FIG. 7, reference numeral 1 denotes a constant current source, reference numeral 2 denotes a differential amplifier, reference numeral 3 denotes a driver transistor, reference numerals 4 and 5 denote output voltage setting resistors, reference numeral 6 denotes a reference voltage source, and reference numeral 7 denotes an output current limiting circuit.
At first, a voltage is applied to a Vin terminal as an input terminal while a load is connected to a Vout terminal as an output terminal. Then, a power is supplied to the differential amplifier 2. A power is also supplied to the reference voltage source 6 via the Vin terminal and the constant current source 1, respectively. A change of the output voltage between a GND terminal and the Vout terminal is detected by a voltage dividing circuit composed of the output voltage setting resistors 4 and 5, then fed back towards the differential amplifier 2. The voltage fed back to the differential amplifier 2 is inputted to a positive pole terminal of the differential amplifier 2.
A voltage signal from the reference voltage source 6 is also inputted to a negative pole terminal of the differential amplifier 2. Then the level of each inputted signal is compared. The differential amplifier 2 then amplifies a differential voltage, which is the result of comparison. According to this amplified signal, the driver transistor 3 is controlled so as to eliminate change of the voltage output from the Vout terminal. Here, the output current limiting circuit 7 provided for each of conventional constant voltage regulators limits an amount of the current flowing in the driver transistor 3 so as to block the flow of any excessive current to the Vout terminal, for example, even when the Vout terminal is short-circuited with the GND terminal.
Any of the conventional regulator techniques, however, cannot supply a constant voltage to a load, since the regulator is provided with an output current limiting circuit, thereby the output voltage might drop due to a capacity of the load when the load is activated. This is a problem to be solved.
Under such circumstances, it is an object of the present invention to provide a constant voltage regulator that can supply a constant voltage to a load when the load is activated.
In order to solve the above problem, the constant voltage regulator of the present invention, which is provided with an overcurrent preventing unit for preventing an overcurrent flowing in a load at a time of overload level, is further provided with controlling unit for turning off the overcurrent preventing unit when the load is activated.
Furthermore, it is another object of the present invention to provide a method for controlling the constant voltage regulator provided with overcurrent preventing unit for preventing an overcurrent flowing in the load at a time of overload level, and a controlling unit for turning on/off the overcurrent preventing unit. The controlling unit turns off the overcurrent preventing unit when the load is activated by the controlling unit.
It is still another object of the present invention to provide an electric device provided with the constant voltage regulator as described above.